1. The History of Life on Earth

2. Earth’s Early History How did life on Earth begin?
How did life arrive on a lifeless planet?
Scientists who seek to answer these questions must look for clues that were left behind.

3. Earth’s Early Land Environment
Scientists speculate that Earth formed 4.6 billion years ago. What do you think?
In it’s “young” age, scientists believe that Earth was a molten body with volcanic features.

4. Earth’s “Early” Atmosphere
No one can be certain of Earth’s early atmosphere.
The gases that made up the atmosphere are believed to have been those expelled by volcanoes.
In contrast to today’s atmosphere, scientists infer that there was very little free oxygen available in Earth’s early atmosphere. Oxygen would break down forming biomolecules.

5. Clues in the Rocks
Rocks may give us clues about what Earth was like in the beginning and what species lived during that time.
To find out when the first organisms formed, scientists study fossils.

6. What is a fossil?
A fossil is a preserved or mineralized remain or imprint of an organism that lived long ago.
Only a small percentage of the species that once lived are believed to be preserved as fossils.
Plants, animals and even bacteria can form fossils.

7. Fossils are believed to rarely occur:
Absence of fossilizing environment.
Lack of hard tissues or parts thought capable of fossilizing.
Body scavenged/decomposed before fossilization.
Nature destroyed the fossil after formation.
The fossil has not yet been discovered.

8. Paleontologists
A paleontologist is a scientist who studies fossils. They use fossils to put together a picture of extinct communities as if they were alive today.
They use three methods to find out the age of rocks.

9. Fossil Dating Techniques
Relative Dating: determines age of rocks by comparing them to rocks in other sediment layers, and assuming lower layers are much older. Law of Superposition: younger layers of rock deposit on top of older layers. Again, how much older? Radiometric Dating: determines the age of rocks by using the decay rate of radioactive isotopes in the rocks. The half-life, relative amount of isotope and the decay product must be known. Several assumptions are made.

10. Geologic Time
A visual representation of the major geological and biological events of Earth’s presumed history.

11. The Origins of Life
Spontaneous Generation: Life from nonlife (Redi’s Experiment?)
Theory of Biogenesis: ONLY living organisms produce living organisms (Pasteur’s Experiment)

12. If life can only arise from pre- existing life, then how did the first life form appear?

13. Modern Ideas for the Origin of Life
SIMPLE MOELCULES IN EARTH’S EARLY ATMOSPHERE COMLEX ORGANIC MOLECULES  PROTEINS and GENETIC CODE  METABOLIC PATHWAYS  PROKARYOTES  EUKARYOTES
Primordial Soup Hypothesis
Miller-Urey Experiment
Endosymbiont Theory

14. Scientists do not know the early steps that led to life on Earth
Scientists do not know the early steps that led to life on Earth. They must continue to ask questions, test theories, and evaluate new evidence as they seek to understand answers about the origin of life.